In a building having a group of elevators, elevator interfloor traffic and traffic from the lobby or main floor (e.g. lobby) to upper floors varies throughout the day. Traffic demand from the main lobby is manifested by the floor destinations entered by passengers (car calls) on the car call buttons. Traffic from the lobby is usually highest in the morning in an office building. This is known as the "up-peak" period, the time of day when passengers entering the building at the lobby mostly go to certain floors and when there is little if any "interfloor" traffic (few hall calls). Within the up-peak period, traffic demand from the lobby may be time related. Groups of workers for the same business occupying adjacent floors may have the same starting time but different from other workers in the building. A large influx of workers may congregate in the lobby awaiting elevator service to a few adjacent or contiguous floors. Some time later, a new influx of people will enter the lobby to go to different floors.
During an up-peak period, elevator cars that are at the lobby frequently do not have adequate capacity to handle the traffic volume (the number of car calls) to the floors to which they will travel. Some other cars may depart the lobby with less than their maximum (full) loads. Under these conditions, car availability, capacity and destinations are not efficiently matched to the immediate needs of the passengers. The time it takes for a car to return to the lobby and pick up more passengers (passenger waiting time) expands when these loading disparities are present.
In the vast majority of group control elevator systems in use, waiting time expansion is traceable to the condition that the elevator cars respond to car calls from the lobby without regard to the actual number of passengers in the lobby that intend to go to the destination floor. Two cars can serve the same floor, separated only by some dispatching interval (the time allowed to elapse before car is dispatched). Dispatching this way does not minimize the waiting time in the lobby because the car load factor (the ratio of actual car load to its maximum load) is not maximized and the number of stops made before the car returns to lobby to receive more passengers is not minimized.
In some existing systems, for instance U.S. Pat. No. 4,305,479 to Bittar et al on "Variable Up-Peak Elevator Dispatching", assigned to Otis Elevator Company, the dispatching interval from the lobby is regulated. Sometimes, this means that a car, in a temporary dormant condition, may have to wait for other cars to be dispatched from the lobby before receiving passengers who then enter car calls for the car.
To increase the passenger handling capacity of time, the number of stops that a car can make may be limited to certain floors. Cars, often arranged in banks, may form a small group of cars that together serve only certain floors. A passenger enters any one of the cars and is permitted to enter a car call (e.g., pressing a button on the car operating panel) only to the floors served by the group of cars. "Grouping", as this is commonly called, increases car loading, improving system efficiency, but does not minimize round trip time back to the lobby. The main reason is that it does not force the car to service the lowest possible floor with the minimum number of stops before reaching that floor.
In some elevators, cars are assigned floors based on car calls that are entered from a central location. U.S. Pat. No. 4,691,808 to Nowak et al on "Adaptive Assignment of Elevator Car Calls", assigned to Otis Elevator Company, describes a system in which that takes place, as does Australian Pat. No. 255,218 granted in 1961 to Leo Port. This approach directs the passengers to cars.